Constraints on the composition and particle sizeof chloride salt-bearing deposits on MarsTimothy D. Glotch1, Joshua L. Bandfield2, Michael J. Wolff2, Jessica A. Arnold3, and Congcong Che1

1Department of Geological Sciences, Stony Brook University, Stony Brook, New York, USA, 2Space Science Institute, Boulder,Colorado, USA, 3Department of Physics, University of Oxford, Oxford, UK

Abstract Chloride salt-bearing deposits onMars were discovered using theMars Odyssey Thermal EmissionImaging System (THEMIS) and have been characterized by both mid-infrared (MIR) and visible-to-near-infrared(VNIR) remote sensing instruments. The chloride salt-bearing deposits exhibit a blue slope at MIR wavelengthsand a featureless red slope at VNIR wavelengths. These deposits also lack strong 3μm bands in VNIR spectra,indicating that they are desiccated compared to the surrounding regolith. The lack of VNIR spectral featuressuggests that an anhydrous chloride salt, the most likely of which is halite, is responsible for the observedspectral slope. In this work, we use laboratory spectra and a hybrid T-matrix/Hapke light scattering model toconstrain the particle sizes and salt abundances of the Martian chloride salt-bearing deposits. Our work showsthat the two broad spectral classes of these deposits observed by THEMIS can be explained by a difference inthe particle size of the admixed silicate regolith. In all cases, chloride salt abundances of 10–25% are required tomatch the THEMIS data. The chloride salt abundances determined in this work suggest deposition in alacustrine/playa setting or in association with late-stage groundwater upwelling.

1. Introduction

Chloride salts have been identified on Mars using the Mars Odyssey Thermal Emission Imaging System(THEMIS) and Mars Reconnaissance Orbiter Compact Reconnaissance Imaging Spectrometer for Mars(CRISM) [Osterloo et al., 2008, 2010; Murchie et al., 2009; Wray et al., 2009; Glotch et al., 2010; Rueschet al., 2012]. They are identified in mid-infrared (MIR) Thermal Emission Spectrometer (TES) and THEMISdata by the presence of a distinctive blue slope, resulting in a steady decrease in apparent emissivity withincreasing wavelength (decreasing wave number) over the 8–12μm spectral range. At visible/near-infrared(VNIR) wavelengths, these same deposits show a featureless red slope relative to the surrounding terrainover the ~1–2.6μm range in CRISM reflectance data. A positive 3μm feature in CRISM and Observatoirepour la Minéralogie, l’Eau, les Glaces et l’Activité (OMEGA) ratio reflectance spectra supports the interpre-tation that the deposits are desiccated compared to the surrounding terrains [Murchie et al., 2009; Glotchet al., 2010; Ruesch et al., 2012]. The apparent desiccation of these terrains is supported by evidence fordesiccation cracks in high-resolution imagery [El-Maarry et al., 2013, 2014].

The interpretation of these deposits as chloride salt-bearing has been somewhat controversial due to the lackof diagnostic spectral absorptions of anhydrous chlorides in both the VNIR and MIR spectral ranges. Despitethis, these materials have unusual spectral properties that are quite distinct from most other materialspresent on planetary surfaces. For example, the distinctive blue slope of chloride deposits in MIR spectra isdue to the low emissivity of chloride salts. Laboratory data also show that the characteristic CRISM spectrafor these regions can be matched by mixtures of anhydrous chloride salts and silicates [Jensen and Glotch,2011; Ruesch et al., 2012].

Here we report on a new spectral class of chloride salt-bearing deposits found on Mars and, for the first time,constrain the abundances and particle sizes of salts and silicates present in these deposits. The new class ofdeposits exhibits the same distinct blue slope in THEMIS data, but their characteristic spectra display a uniqueconcave down spectral shape.

For this work, we acquired MIR emissivity spectra of the suite of silicate/halite mixtures described by Jensenand Glotch [2011]. We use these data, supported by a hybrid light scattering model based on the multiplesphere T-matrix (MSTM) model [Mackowski and Mishchenko, 2011] and Hapke’s thermal emissivity model[Hapke, 1996], to constrain the salt content and silicate particle size of chloride salt-bearing depositson Mars.

GLOTCH ET AL. CHLORIDE SALT-BEARING DEPOSITS ON MARS 454

PUBLICATIONSJournal of Geophysical Research: Planets

RESEARCH ARTICLE10.1002/2015JE004921

Key Points:• Laboratory spectra and scatteringmodels confirm the presence ofchloride salt deposits on Mars

• THEMIS spectra of salt deposits areconsistent with the presence ofapproximately 10–25 wt % halite

• Salts were deposited in either alacustrine/playa setting or bylate-stage groundwater upwelling

Correspondence to:T. D. Glotch,timothy.glotch@stonybrook.edu

Citation:Glotch, T. D., J. L. Bandfield, M. J. Wolff,J. A. Arnold, and C. Che (2016), Constraintson the composition and particle size ofchloride salt-bearing deposits on Mars,J. Geophys. Res. Planets, 121,454–471, doi:10.1002/2015JE004921.

Received 11 AUG 2015Accepted 16 FEB 2016Accepted article online 20 FEB 2016Published online 30 MAR 2016

©2016. American Geophysical Union.All Rights Reserved.

Chloride salt-bearing deposits on Mars occur in a variety of geologic contexts, including possible paleolakes,crater fill, inverted channels, and distributary fans [Osterloo et al., 2008, 2010; Murchie et al., 2009;Wray et al.,2009; Glotch et al., 2010; Ruesch et al., 2012; El-Maarry et al., 2013; Hynek et al., 2015; Osterloo and Hynek, 2015].These studies generally support an interpretation that the deposits formed in an aqueous environment.Considering these data in tandemwith the data presented here and in Jensen and Glotch [2011], who showedthat the near-IR spectral character of the deposits is consistent with halite/silicate mixtures, we conclude thatanhydrous chloride salt, specifically halite, is the only mineral that is both geologically plausible and matchesthe observed spectral properties (transparent with a low emissivity over the required wavelength ranges).

2. Data and Methods2.1. THEMIS Data Analysis

Osterloo et al. [2010] identified 641 individual chloride salt deposits covered by several hundred high-qualityTHEMIS daytime images. While an exhaustive spectral survey of each of the sites identified by Osterloo et al.[2010] is impractical, we used the Java Mission-planning and Analysis for Remote Sensing (JMARS) software[Christensen et al., 2009] to examine 10 occurrences of the chloride salt deposits in high-quality THEMISimages, based on maximum surface temperature, for detailed spectral analysis. THEMIS images were atmo-spherically corrected using the methods of Bandfield et al. [2004a]. These images, the associated center lati-tudes and longitudes of the chloride salt units and minimum and maximum surface temperatures, are listedin Table 1. To summarize briefly, we (1) applied a correction to remove a constant radiance contribution dueto atmospheric emission, (2) retrieved surface emissivity using low spatial resolution, but high spectralresolution TES data covering the region of interest, (3) mapped the derived surface emissivity to THEMIS datato derive the atmospheric attenuation of surface emitted radiance, and (4) corrected for this atmosphericcomponent to derive surface emissivity for the entire image.

2.2. Sample Selection and Preparation

Our samples consist of physical mixtures of halite and flood basalt. The basalt was purchased from Ward’sScience and is from the Columbia River Plateau. The halite was acquired from Acros Organics (reagent grade99%+ synthetic NaCl). The flood basalt and halite samples were crushed with a steel mortar and pestle anddry sieved to produce a range of coarse size fractions. To produce a<10μm size fraction, samples were sepa-rated in ethanol using Stokes’ settling method [Day, 1965; Gee and Bauder, 1986; Salemi et al., 2010].Subsequent to settling, samples were dried at 80°C. For this work, we use four size fractions from Jensenand Glotch [2011]: 250–355, 125–180, 63–90, and< 10μm. The samples and preparation methods aredescribed in detail by Jensen and Glotch [2011]. Samples are currently stored in the Stony Brook UniversityVibrational Spectroscopy Laboratory and are available upon request.

2.3. Laboratory Spectroscopic Measurements

We heated each sample to 80°C overnight prior to spectral measurements. We collected emissivity spectra ofeach sample over the 2000–200 cm�1 (5–50μm) spectral range using a Nicolet 6700 Fourier transforminfrared (FTIR) spectrometer modified to measure sample emitted radiance. Each spectrum is an averageof 256 scans and was calibrated using a blackbody target heated to 70°C and 100°C in the manner of Ruffet al. [1997].

Table 1. Image IDs and Ancillary Information for the 10 THEMIS Images Analyzed for This Study

Image ID Center Longitude Center Latitude Minimum Temperature (K) Maximum Temperature (K)

I33927002 178.01 -34.35 249.30 299.78I35565002 91.50 -18.72 214.55 277.78I36355001 343.74 -11.74 198.17 266.35I39307007 113.79 -2.27 217.99 278.39I39643002 166.80 -18.32 221.56 276.10I40123002 357.46 -31.98 234.72 287.03I41186002 291.00 -34.94 262.06 304.67I41227002 190.68 -21.55 246.90 302.04I41850005 216.33 -33.30 263.02 307.38I42789003 129.24 -30.47 243.39 287.37

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Note that our laboratory spectra are converted to emissivity in a slightly different manner than theremotely sensed emissivity spectra of the Martian surface from TES. TES spectra are converted toemissivity by dividing radiance spectra by a blackbody of the highest brightness temperature determinedwithin a 50 cm�1 band near 1300 cm�1 [Bandfield et al., 2000]. Our laboratory emissivity spectra werecalculated in a similar manner, although the position of the maximum brightness temperature was notconstrained. The result is that some of our laboratory spectra do not exhibit the overall slopes andemissivity values> 1.0 seen in TES spectra of Martian chloride salt deposits [e.g., Osterloo et al.,2008, Figure 3b].

2.4. Modeling Emissivty Spectra and NaCl Optical Constants

We utilized the multiple sphere T-matrix (MSTM) model of Mackowski and Mishchenko [2011] to exactlycalculate the phase function and “near-field” scattering properties of clusters of closely packed mineralgrains (halite and silicate). The goals of this modeling effort are to (1) provide a theoretical frame of refer-ence for the laboratory measurements, (2) calculate Mars surface spectra using actual Mars remote sen-sing data (Mars dust optical constants from Wolff et al. [2006]) to provide a more direct comparisonbetween Martian data and laboratory analyses, and (3) investigate how the increase in abundance oftransparent chloride salt alters scattering parameters such as the phase function. The fundamental inputsinto this model are the real and imaginary indices of refraction, n and k, of the materials of interest andthe position and size parameter (x ¼ 2π

r where r is the radius of the particle) of each sphere in the cluster.

For each sphere in the cluster, the incident and scattered fields can be represented by regular and out-going vector spherical wave function expansions, centered about the origin of the sphere. The sum ofthe scattered fields for each sphere represents the scattered field of the cluster as a whole. The mathema-tical formulation and details of code implementation are given by Mackowski and Mishchenko [1996,2011].

We calculate hemispherical emissivity according to equation (64) of Hapke [1996], which approximates theemissivity of a surface heated from below and is appropriate for comparison to our laboratory measurements:

ϵh ¼ 2γ= ζ þ γð Þ; (1)

where

γ ¼ffiffiffiffiffiffiffiffiffiffiffiffi1� w

p; (2)

and ζ ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1� β�w

p: (3)

Here β is the hemispherical asymmetry parameter, γ is the thermal albedo factor, and ζ is the thermalasymmetry factor. The MSTM code calculates the asymmetry parameter of the cluster phase function, g,according to

g ¼ 1=2∫d cosθð Þp θð Þcos θð Þ ¼ cos θð Þh i ¼ u; (4)

where θ is scattering angle and p(θ) is the single-scatting particle phase function [Mishchenko, 1994]. Usingthe Sagan-Pollack approximation [Sagan and Pollack, 1967; Wiscombe and Grams, 1976], β ≈ g, we replace βin equation (3) with g, which is derived from the MSTM calculation.

The MSTM code also calculates the scattering efficiency, Qsca, and total extinction, Qext, providing additionalinput parameters for our spectral analysis. At each wavelength, we calculate the single-scattering albedoaccording to

w ¼ Qsca=Qext: (5)

To model finely particulate chloride salt-bearing deposits on Mars, we used a sphere cluster consisting of377 randomly close-packed (packing fraction of ~0.6) 10μm diameter spheres generated using a moleculardynamics-based Fortran code described in Donev et al. [2005] and available at http://cims.nyu.edu/~donev/Packing/PackLSD/Instructions.html. The number of particles in the cluster represents a compromisebetween achieving a large enough number of particles to reasonably model a regolith surface and thecomputational cost of the calculations, which increases as the cube of the cluster size parameter. Eachsphere in the cluster was assigned either the optical constants of Mars dust (derived by Wolff et al.[2006]) or halite (derived specifically for this work), and the model was run at roughly 20 cm�1 intervals

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between 400 and 1538 cm�1 to buildthe spectra. We varied the percentageof halite in the model between 0 and95% to assess the effects of halite abun-dance on the resulting calculated mid-IR spectra.

To model the halite refractive indices,we used the Stony Brook UniversityVibrational Spectroscopy Laboratory’sNicolet 6700 FTIR spectrometer to acquirea specular reflectance spectrum of apressed pellet of reagent-grade halite.The mid-IR spectrum (2000–400 cm�1)was acquired using a deuterated L-alanine doped triglycine sulfate (DLaTGS)detector with a KBr window. A far-IR spec-trum (600–100 cm�1) was acquired usinga DLaTGS detector with a polyethylenewindow. The far-IR spectrum was scaleddown by ~3% to account for slight differ-ences in detector linearity over the over-lapping wavelength range and mergedwith the mid-IR spectrum at ~500 cm�1.The mid-IR spectrum was an average of256 scans. Due to a lower signal-to-noiseratio for the far-IR detector, we averaged1024 scans to construct the far-IR spec-trum. We then used Lorentz-Lorenz dis-persion theory [Spitzer and Kleinman,1961] to determine the real and imagin-

ary refractive indices of halite from the merged spectrum. Details of the model can be found in Glotch et al.[2007] and Glotch and Rossman [2009]. Halite optical constants and dispersion parameters are shown inFigure 1 and Table 2, respectively, and are archived at http://aram.ess.sunysb.edu/optical_constants.html.

3. Results3.1. THEMIS Data

For each of the 10 chloride salt locations chosen for this study, we confirmed the detections of Osterloo et al.[2010] by creating decorrelation-stretched (DCS) images [e.g., Gillespie et al., 1986] utilizing THEMIS bands 8,7, and 5 as red, green, and blue (Figure 2). With this band combination, chloride salts typically appear brightblue or bluish-green in the THEMIS DCS images [Osterloo et al., 2008, 2010; Glotch et al., 2010]. The chloride saltsites identified in this study generally consist of rough, irregular, visibly light toned patches on 1–10 km scales,superposing the plains on which they were deposited, often, though not always, in local topographic lows. Intwo cases (Figures 2g and 2i), the chloride salt deposits are associated with inverted channel topography

features, suggesting as association withat least some flowing surface water.

At decameter spatial scales, substantialgeomorphologic differences betweenthe deposits are observed. For example,Figure 3a shows an irregular distributionof salty patches over the local heavilycratered terrain. In this instance, chloridesalt deposits appear to cover both local

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Figure 1. Refractive indices of Mars dust [Wolff et al., 2006] and halite(generated in this work) used in our scattering model. (a) Real index ofrefraction. (b) Imaginary index of refraction.

Table 2. Dispersion Parameters for Halitea

ν (cm�1) γ 4πρ

165 0.0361 3.3793212 0.1869 0.0440233 0.0767 0.0396254 0.0825 0.0362

aε0 = 2.5576.

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topographic highs and lows, and there appears to be little correlation with fluvial features. By contrast,comparatively smaller deposits are shown in Figures 3b and 3c where chloride salt deposits occur in localtopographic lows. In Figure 3b, the chloride salt deposit has been reworked into aeolian bed forms. No suchbed forms are obvious in the deposit shown in Figure 3c. However, this deposit (also shown in Figure 2i) is

Figure 2. THEMIS DCS images showing the 10 chloride salt sites characterized in this study. For each image, bands 8, 7, and 5 are assigned to red, green, and blue,respectively. Locations for each chloride salt unit are given in Table 1. (a) I33927002. (b) I35565002. (c) I36355001. (d) I39307007. (e) I39643002. (f) I40123002.(g) I41186002. (h) I41227002. (i) I41850005. (j) I42789003. White boxes in Figures 2a, 2e, and 2i indicate the positions of Figures 3a–3c. Deposits in Figures 2d, 2e, and2h occur in moderately dusty regions, resulting in anomalous THEMIS spectra displayed in Figure 4.

Figure 3. THEMIS DCS images overlaid on Context Camera (CTX) imagery. Blue colors indicate the presence of chloride salt. Salts typically occur in local topographiclows and are associated with rough, high-albedo patches. (a) THEMIS image I33927002 overlaid on CTX image D17_033885_1453_XN_34S182W. (b) THEMIS imageI39643002 overlaid on CTX image B18_016585_1635_XN_16S193W. (c) THEMIS image I41850005 overlaid on CTX image P12_005625_1470_XI_33S143W. Thedeposit in Figure 3b occurs in a moderately dusty region, resulting in one of the anomalous spectra shown in Figure 4.

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clearly associated with inverted channeldeposits, suggesting water flowing intoand out of the local topographic low inwhich the chloride salt-bearing depositsits. This in turn suggests that watermay have ponded at the surface in thisregion for a period of time.

Our spectral survey of the 10 chloridesalt locations chosen for this study(Figure 4) found a range of spectralshapes; all of which display a blue slopebetween 800 and 1200 cm�1. Mostdisplay superposed silicate absorptionscentered between 1000 and 1100 cm�1

that are consistent with the spectralshapes of the local basaltic regolith.However, at three locations (Figures 2d,2e, and 2h), which have low to moderatedust cover index (DCI) values (Figure 5),we found that the average THEMIS spec-tra display a blue slope, but with highoverall emissivity and a concave down

spectral shape between 900 and 1200 cm�1. Each of these three surfaces has an emissivity maximum in band4 (1175 cm�1 and 8.51μm) and a steeply dipping emissivity toward higher wave numbers/shorter wave-lengths, consistent with multiple scattering by fine particulates. Some spectra have maximum emissivitiesgreater than unity as a result of an atmospheric correction artifact. In all cases, surface emissivity is chosento be unity in the band of highest brightness temperature. This typically occurs in band 3 or occasionally 9(bands 1 and 2 are excluded) because the atmospheric dust is most transparent in these bands. If bands 4or 5 have a higher apparent surface emissivity, the dust more than compensates for that, and those bandsstill appear with lower brightness temperatures in the uncorrected spectra.

3.2. Laboratory Spectra

Spectra of the particulate mixtures are shown in Figure 6. The 250–355μm size fraction with 1–10% halite(Figure 6a) displays emissivity spectra typical for basalt. Spectra of these samples have broad absorptionsfrom ~1200–600 cm�1 and ~700–300 cm�1, similar to the TES Surface Type 1 (ST1) [Bandfield et al., 2000]or Group 3 spectrum [Rogers and Christensen, 2007]. The 1 and 5% halite spectra are nearly identical, withemissivity maxima of unity at 1252 cm�1 and comparable spectral contrast. At 10% halite content, the overall

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Figure 4. Average atmospherically corrected THEMIS surface emissivityspectra of chloride salt-bearing deposits. Typical spectra (black) have avariety of shapes, but all display blue slopes and absorptions between1200 and 900 cm�1. Spectra from the three anomalous deposits (red)display similar blue slopes but have emissivity maxima between 1200 and900 cm�1. Typical errors for the spectra displayed range between 0.005and 0.01 emissivity units.

Figure 5. Osterloo et al. [2010] chloride salt occurrence database (white polygons) overlaid on TES dust cover index (DCI)[Ruff and Christensen, 2002]. The DCI is displayed from values of 0.92 (red) to 1.00 (purple). Three sites in moderate dustcover (circles marked by white arrows) display anomalous spectral character in THEMIS data. DCI values for the threechloride salt deposits are 0.959 (image I39643002), 0.964 (image I39307007), and 0.947 (image I41227002).

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GLOTCH ET AL. CHLORIDE SALT-BEARING DEPOSITS ON MARS 459

spectral contrast is reduced slightly, and the emissivity maximum in the Christiansen feature (CF) region shiftsto 1245 cm�1, with an emissivity of 0.995. At 25% halite content, the spectral contrast is further reduced, andthe emissivity maximum shifts to 1240 cm�1 with a maximum emissivity in the CF region of 0.99. For both the10 and 25% halite spectra, the laboratory calibration code determined that the maximum brightnesstemperatures (where emissivity is taken to be unity) occurs at ~1640 cm�1, far from the typical CF regionfor silicates. The spectra drastically change for the 50% and 75% halite spectra, which have broad emissivitymaxima between ~1200 and 600 cm�1, where the other spectra have absorptions. Over the 1600–600 cm�1

range, the maximum emissivity for the 50% halite spectrum is 0.98, while for the 75% halite spectrum, it is0.94. The emissivity maxima for these spectra occur at 313 cm�1.

The trends seen in the 125–180μm size fraction spectra (Figure 6b) are similar to those seen in the 250–355μmsize fraction, although more pronounced. Again, the 1% and 5% halite spectra are very similar, with identicalemissivity maxima of unity at 1243 cm�1. The 5% halite spectrum has a slightly reduced spectral contrast, witha minimum emissivity between 1200 and 600 cm�1 of 0.97, compared to a value of 0.96 for the 1% emissivityspectrum. At 10% halite, the maximum emissivity in the CF region is reduced to 0.995, while the absolutemaximum emissivity is unity at 651 cm�1. At 25% halite, the spectral contrast shallows considerably, and theemissivity maximum in the CF region shifts from 1243 cm�1 to 1148 cm�1, with a maximum value of 0.994.The broad emissivity minimum present for the lower halite contents is nearly gone, replaced by an invertedReststrahlen band between ~1150 cm�1 and 820 cm�1. The 50% halite spectrum has a broad, concave downfeature between ~1300 cm�1 and 820 cm�1, with a maximum emissivity of 0.96. The 75% halite spectrum issimilar, with a maximum emissivity of only 0.90 in the same spectral range. The absolute maximum emissivitiesfor each of the 25%, 50%, and 75% halite spectra occur at ~313 cm�1, with values of unity.

At a size fraction of 63–90μm (Figure 6c), differences from the coarser size fractions are apparent. For halitecontents less than 10%, the spectra are comparable, with each having an emissivity maximum of unity at1243 cm�1 and with a reduction in spectral contrast with increasing halite content. The 10% halite spectrum

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Figure 6. Laboratory emissivity spectra of basalt-halite mixture powders. (a) 250–355 μm size fraction. (b) 125–180 μm sizefraction. (c) 63–90 μm size fraction. (d)< 10 μm size fraction.

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also displays a slight decrease in emissiv-ity between 1243 and 827 cm�1, whichis absent at lower halite contents. At25% halite content, the CF shifts to1142 cm�1, and the spectrum displays astrong blue slope with low spectralcontrast between 1142 and 827 cm�1.The 50 and 75% halite spectra displaydramatic differences from the lowerhalite content spectra, with stronglyconcave down features between 1300and 819 cm�1 and maximum emissivitiesof 0.98 and 0.93, respectively.

The< 10μm size fraction (Figure 6d) dif-fers substantially in its spectral behaviorcompared to all of the coarser size frac-tions. For halite contents of 1 and 5%,the spectra have CF positions of 1256and 1243 cm�1, respectively, and displaya blue slopes with low spectral contrast,between the CF position and 820 cm�1.The 10% halite spectrum shows the samegeneral behavior, but with a CF positionof 1237 cm�1 and a maximum emissivityin the CF region of 0.98. The 25% halitespectrum displays a CF at 1140 cm�1,with a minimum emissivity of 0.99, and aslight concave down shape between1300 and 820 cm�1 that is more pro-nounced in the higher halite contentspectra. At 50 and 75% halite contents,the spectra display strong concavedown spectral shapes between 1300 and820 cm�1, with maximum emissivities of0.98 and 0.97, respectively.

3.3. Model Results

Modeled spectra for the cluster of 10μmhalite and Mars dust particles are shown

in Figure 7. The pure Mars dust spectrum (0% halite) is consistent with typical TES bright region spectra ofMars, with a CF position of 1238 cm�1 and a maximum emissivity of 0.995. The spectrum includes a broad,shallow, concave up spectral feature between 938 and 1238 cm�1, and the prominent silicate transparencyfeature at 839 cm�1 is well modeled. At 10% halite content, the CF is shifted to 1218 cm�1, and the maximumemissivity is reduced to 0.987. The broad central feature is considerably shallowed, although it is still slightlyconcave up. At 25% halite, the maximum emissivity is reduced to 0.985, and the CF is shifted to 1138 cm�1.The overall shape of the broad central feature is concave down, although there is still a local minimum with ashallow concave up profile centered at 1078 cm�1. The emissivity of the 25% halite spectrum at most wave-lengths is slightly lower than the 10% halite spectrum at most wavelengths. At 50% halite content, the regionbetween 1238 and 919 cm�1 is strongly concave down, with a maximum emissivity of 0.983 at 1118 cm�1.The emissivity of the spectrum is substantially reduced compared to lower halite contents. Further dramaticchanges to the spectra are seen for 75 and 97% halite contents. Both spectra are dominated by a stronglyconcave down broad feature centered at 1078 cm�1. The maximum emissivities of these spectra are 0.97and 0.87, respectively.

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Figure 7. Modeled spectra of Mars dust/halite mixtures. (a) Full spectracovering the 399–1538 cm�1 (6.5–25 μm) range with halite contentsranging from 0% to 97%. Increasing halite content leads to an overallreduction in emissivity, a shift of the CF to lower wave numbers (longerwavelengths), and a change in spectral shape in the middle portionof the spectrum from slightly concave up to strongly concave down.(b) Same spectra as in Figure 7a, zoomed into the 800–1300 cm�1

region, highlighting the gradual change in spectral shape withincreasing halite content.

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Figure 8 shows the scattering phase functions and associated asymmetry parameters at 1018.5 cm�1 (9.8μm)for each of the clusters we modeled in MSTM. This chosen frequency (1018.5 cm�1) is near the center of the

broad feature in our laboratory andmodeled spectra that originates as ashallow concave up feature and invertsto a strongly concave down feature withincreasing halite content (Figures 6and 7). As with the spectra shownin Figure 7, there is little differencebetween the scattering phase functionsand asymmetry parameters for the 0%,10%, and 25% halite clusters, especiallyat low-scattering angles. Between ~120and 180°, the phase function curvesseparate a bit, with higher haliteabundance clusters being slightly lessforward scattering (lower asymmetryparameters) than clusters with lowerhalite abundances. At 50% halite abun-dance, the phase function starts todivert from the lower halite abundancecurves, even at phase angles as low as30°. At 75% and 97% halite abundances,the phase curves divert substantiallyfrom the lower halite contents, withgenerally higher magnitudes at scatter-ing angles higher than 50°. An exceptionis that the 75% halite content phasecurve has a lower magnitude thanthe 0 and 10% halite content curvesbetween ~125 and 160°.

Figure 8. Scattering phase functions derived fromMSTM for the halite/dust clusters modeled in this work. S11 denotes thatthe phase function is the (1,1) element of the Stokes scattering matrix.

Figure 9. Near-field visualization of the electric field magnitude, |E|2,surrounding clusters of Mars dust and halite particles at 1018.5 cm�1

(9.8μm). (a) 10% halite. (b) 25% halite. (c) 50% halite. (d) 97% halite. Whitearrows indicate spheres with higher internal E field magnitudes thansurrounding spheres. As the halite component of the cluster is increased,the clusters become more transparent, resulting in higher electric fieldmagnitudes in the cluster interiors.

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We used MSTM to gain additional insight into the effects of increasing transparency and changing phasefunction due to increased halite content. In our model, we calculate the near-field electric field magnituderelative to the magnitude of the field of the incoming beam, |E|2, surrounding and within our cluster ofspheres at 1018.5 cm�1 (9.8μm) for 10, 25, 50, and 97% halite content. The E field magnitude can be thoughtof as a proxy for scattering, such that high magnitudes within the cluster indicate transparency at the chosenwavelength, while low magnitudes indicate high opacity. The results of these near-field models are shown inFigure 9, which depicts |E|2 in a plane taken through the center of the modeled clusters. At 10 and 25% halitecontent (Figures 9a and 9b), the electric field magnitude at the center of the cluster is at or close to 0,indicating that the cluster is opaque at this wavelength. At 25% halite, several spheres can be seen withlow (~0.17–0.32 V2/m2) but nonzero electric field magnitudes (white arrows in Figure 9b), indicatingincreased transmission of light through the cluster. At 50% halite, the cluster is clearly more transparent, withmultiple spheres with nonzero electric field magnitudes toward the center of the cluster (white arrows inFigure 9c). As would be expected, the 97% halite cluster is nearly completely transparent, with the centersphere (white arrow in Figure 9d) in the cluster displaying electric field magnitudes ~1.8 V2/m2 and numerousother spheres in the cluster exhibiting high E field magnitudes, indicating their transparency.

4. Discussion4.1. Comparison of Laboratory and Modeled Spectra to THEMIS

To directly compare our laboratory and modeled spectra to THEMIS data of chloride salt-bearing deposits onMars, we resampled them to THEMIS spectral sampling using the THEMIS filter functions. Convolvedlaboratory spectra are shown in Figure 10, and convolved modeled spectra are shown in Figure 11.Emissivity maxima and calculated slopes for the THEMIS data are shown in Figure 4, and the laboratory dataconvolved to THEMIS spectral sampling are shown in Table 3. The THEMIS data fall into two broad spectral

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Figure 10. Laboratory spectra of halite/basalt mixtures convolved to THEMIS spectral sampling and compared withrepresentative THEMIS spectra of chloride salt-bearing surfaces on Mars. Vertical lines indicate the position of THEMISband 4. (a) 250–355 μm size fraction. (b) 125–180 μm size fraction. (c) 63–90 μm size fraction. (d) <10 μm size fraction.THEMIS spectra taken from chloride salt deposits in dustier regions of Mars.

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classes (Figure 4) that are strongly con-trolled by particle size and chloride saltabundance. Laboratory data convolvedto THEMIS spectral sampling constrainthe particle sizes and chloride saltabundances that give rise to these twospectral classes.

As suggested by the full-resolutionlaboratory data, the coarsest size frac-tion (250–355μm; Figure 10a) mixturesgenerally do not match representativeTHEMIS spectra of chloride salt-bearingsurfaces at any salt content. At lowerhalite contents, the spectra do not havethe blue slope that is characteristic ofthe THEMIS data. At the highest halitecontents, the overall emissivity is muchtoo low and the magnitude of theinverted silicate feature is much too

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Table 3. Emissivity Maximum Positions and Calculated Slopes for the THEMIS Spectra Shown in Figure 4 and the Basalt/Halite Mixture Spectra Resampled to the THEMIS Spectral Bands

THEMIS Image/Sample Emissivity Maximum (μm) 8.51–12.56 μm Slope (× 10�3)

I33927002 7.89 3.58I35565002 7.89 7.23I36355001 8.51 6.86I39307007 8.51 9.56I36943002 8.51 9.38I40123002 8.51 4.74I41186002 7.89 2.90I41227002 8.51 6.94I41850005 8.51 2.68I42789003 7.89 5.24250–355 μm, 1% halite 7.89 -0.28250–355 μm, 5% halite 7.89 -0.58250–355 μm, 10% halite 7.89 -0.01250–355 μm, 25% halite 7.89 0.31250–355 μm, 50% halite 7.89 0.37250–355 μm, 75% halite 10.17 -0.69125–180 μm, 1% halite 7.89 -0.81125–180 μm, 5% halite 7.89 0.84125–180 μm, 10% halite 7.89 -1.45125–180 μm, 25% halite 8.51 1.72125–180 μm, 50% halite 9.30 1.47125–180 μm, 75% halite 10.17 0.3763–90 μm, 1% halite 7.89 1.9663–90 μm, 5% halite 7.89 1.8163–90 μm, 10% halite 8.51 2.0263–90 μm, 25% halite 8.51 2.0363–90 μm, 50% halite 9.30 1.5663–90 μm, 75% halite 10.17 2.55<10 μm, 1% halite 8.51 5.66<10 μm, 5% halite 8.51 6.96<10 μm, 10% halite 8.51 3.61<10 μm, 25% halite 9.30 4.90<10 μm, 50% halite 9.30 5.63<10 μm, 75% halite 9.30 2.66

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large to match the THEMIS data. The trend seen in the 125–180μm size fraction (Figure 10b) is similar forlower halite contents. Up to 25% halite, the spectra do not have blue slopes. However, at 25% halite, thespectral character changes markedly, as the spectral contrast is greatly reduced, and the spectrum displaysa prominent blue slope and low spectral contrast, similar to that seen in some THEMIS data. At 50% halite,the overall emissivity and spectral shape are poor matches to the THEMIS data. For the 63–90μm size fraction(Figure 10c), the blue slope is present at 10% salt content and becomes more prominent at 25% halite. Inboth cases, these spectra resemble THEMIS data of the Martian low-albedo chloride salt-bearing sitesdiscussed by Osterloo et al. [2008, 2010] and Glotch et al. [2010]. As with the coarser size fractions the 50%halite spectrum is a poor match to the THEMIS data. Finally, the finest size fraction (<10μm) spectra all haveconcave down spectral shapes and do not match typical THEMIS spectra of chloride salt-bearing surfaces.However, these spectra are broadly similar to THEMIS spectra of chloride salt-bearing surfaces in moderatelydusty regions of Mars (Figure 10d). For this size fraction, the range of halite contents (1–25%) providingreasonablematches to the THEMIS spectra is broader, with an emissivitymaximum in the same channel. Aswiththe coarser size fractions, high halite contents (50–75%) result in spectra with low emissivities that are notcomparable to the THEMIS data and a mismatch in the wavelength where the emissivity maximum occurs.

The modeled spectra of 10μm spheres (Figure 11) are generally similar to the finest size fraction laboratoryspectra and the THEMIS data covering high-albedo chloride salt-bearing deposits, with a key difference. Ingeneral, the concave down spectral shape observed in the THEMIS and laboratory data are replicated.However, the spectra are relatively flat rather than sloped through the ~8–11μm region. As seen in thelaboratory data, high halite contents (50–75%) lead to low emissivity spectra that are not consistent withTHEMIS data.

The data presented here indicate that both the particle size and the salt content exert a strong influence onspectra of chloride salt-silicate mixtures, even at the relatively coarse spectral resolution of THEMIS. For coarseparticulates and low halite contents (<10%), spectra of halite-silicate mixtures have an emissivity maximumat 1268 cm�1 (7.89μm; THEMIS band 3). For all but the coarsest particulates, as halite content increases to10–25%, the emissivity maximum shifts to 1175 cm�1 (8.51μm; THEMIS band 4). This shift is generallycoincident with the appearance of a blue slope in the spectra that is characteristic of chloride salt-bearingsurfaces on Mars. As the halite content increases further, to 50–75%, the emissivity maximum shifts furtherto 1076 cm�1 (9.30μm; THEMIS band 5) or 984 cm�1 (10.17 μm; THEMIS band 6), and the spectral emissivitydrops substantially. To date, this extreme shift in the emissivity maximum has not been observed on Mars.

4.2. Constraining Chloride Salt Abundance and Particle Size From THEMIS Data

A key parameter that may help constrain the chloride salt content of the deposits is the position of the emis-sivity maximum in their THEMIS spectra. As seen in Figures 10 and 11, low chloride salt contents (~0–10%)generally result in spectra with an emissivity maximum in THEMIS band 3, while higher chloride salt contents(~10–25%) result in spectra with an emissivity maximum in THEMIS band 4. Laboratory and model data, how-ever, show that the position of the emissivity maximum is also influenced by particle size, with the coarsest par-ticulates maintaining a band 3 emissivity maximum, even for large salt contents. Fortunately, THEMIS data canbe used to assess particle size, both qualitatively and quantitatively. The slope between THEMIS bands 1/2 at6.77μm and band 3 at 7.89μm can be used to characterize the “silicate roll-off” in spectra caused by multiplescatterings due to fine particulates at short wavelengths. In general, fine particulates exhibit a steep roll-off inemissivity at wavelengths short of the Christiansen feature, while coarser particulates exhibit a shallow roll-off orlack one entirely. In Figure 4, the black spectra, typical of chloride salt deposits in less dusty regions of Mars,exhibit relatively shallow slopes between bands 1/2 and band 3, suggesting minimal multiple scattering. Bycontrast, the red spectra, acquired from chloride salt deposits found in duster terrains on Mars, exhibit steeperslopes, suggesting that the band 4 emissivity maxima in spectra of these deposits are at least partially due tothe presence of fine particulates.

The spectra of the high-albedo chloride-bearing units are also similar to spectra of optically thin dust coatingson surfaces that occur when the dust is colder than the atmosphere [Ruff et al., 2006; Hamilton and Ruff, 2012;Rivera-Hernandez et al., 2015]. Both surfaces can have a characteristic convex down shape of the ~8–12μmsilicate Restrahlen absorption due primarily to scattering (for chlorides) or emission (for dust coatings). Incontrast to the chlorides, the convex downward spectral shape due to dust coatings can be caused by astrong temperature contrast between the dust and the substrate. There is little evidence for the presence

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of these effects, especially of this magnitude in the THEMIS data. Adjacent surfaces with similar surfacetemperatures and thermophysical properties as the chloride unit have the spectral properties of surface dustwithout the unique convex down spectral shape. Despite this, there remains the possibility that the spectralfeatures are due to dust coating effects rather than the presence of chlorides. However, given the uniquenessof the spectral features combined with the ubiquity of dusty surfaces on Mars with similar thermophysicalproperties, we believe this to be unlikely.

Thermal inertia can be derived from THEMIS data [Fergason et al., 2006; Christensen and Fergason, 2013] tohelp constrain whether the emissivity maximum position of a spectrum is influenced more by particle sizeor chloride salt content. For instance, the THEMIS spectrum of the chloride salt-bearing deposit from imageI33927002 (Figures 2a and 10) has a blue slope but an emissivity maximum in band 3, implying a chloride saltcontent below ~10%. The thermal inertia of this deposit is 301.1 ± 16.1 Jm�2K�1 s�½, corresponding to aneffective particle size of ~730μm [Presley and Christensen, 1997], which is coarse sand. By contrast, theTHEMIS spectrum from I40123002 (Figures 2f and 10) has both reduced spectral contrast and emissivitymaximum at band 4, which is consistent with a higher chloride salt content (up to 25%). The thermal inertiaof this deposit is 309.9 ± 33.2 Jm�2K�1 s�½, corresponding to an effective particle size of ~824μm, which isalso coarse sand and only slightly different from the previous deposit. No difference between the spectraof these deposits is expected due to the particle size difference, as both particle sizes are much larger thanthe wavelength of light used to investigate the surfaces. In this case, the thermal inertias of the two depositsstrongly suggest that the differences between their spectral properties are caused by chloride salt abun-dance rather than particle size. It should be noted that the thermal inertias of the chloride salt deposits exam-ined in this study and that of Osterloo et al. [2010] generally suggest a coarser particle size than the laboratoryspectra we present here. While thermal inertia is often taken as a proxy for the effective particle size of thesurface, it can also indicate induration. Thermal inertia is sensitive to the physical properties of the top fewcentimeters of the surface [Putzig and Mellon, 2007], while thermal infrared spectroscopic measurementsare only sensitive to the properties of the top ~100μm. We suggest that the discrepancy between themeasured thermal inertias and the laboratory spectroscopic measurements can be explained by chloride saltdeposits that are well indurated but friable, resulting in a more finely particulate surface layer. Such asituation could explain both the thermophysical characteristics and the spectral measurements.

Clearly, particle size exerts a strong control on the spectra of chloride salt-silicate mixtures. For the coarsestparticulates (greater than hundreds of microns), spectral changes are not apparent, even up to 25% chloridesalt content. For these mixtures, surface scattering is the dominant process, and the transparency of thechloride salt does not result in any additional volume scattering. As particle size is reduced, multiple scatter-ings become more evident, and the transparent chloride salt results in multiply scattered radiation far fromthe surface to contribute to the overall signal (Figure 9). This effect is most evident for the finest particulates,which show a concave down spectrum with a spectral shape similar to a transmission spectrum. Modelresults (Figures 7–9) confirm the hypothesis that increased chloride salt content results in increased multiplescatterings from the interior of the cluster. The addition of chloride salt to the cluster also increases the single-scattering albedo at every wavelength. This effect, combined with the lower asymmetry parameters asso-ciated with higher chloride salt contents (Figure 8), results in drastic changes to the shapes and lower overallemissivities of the resulting spectra.

The new class of chloride salt-bearing deposits shown here (Figure 4) is spectrally dissimilar to the THEMISspectra of chloride salt-bearing deposits shown in previous works [Osterloo et al., 2008, 2010; Glotch et al.,2010]. This work shows, though, that the best explanation for this spectral behavior is that these depositsare composed of finely particulate silicates (< ~10μm) intimately mixed with a low emissivity, transparentmaterial such as anhydrous chloride salt. Therefore, the primary difference between this new class of depositsand those shown in previous works is that the silicate component of these deposits is composed of substan-tially finer particulates. This is consistent with their locations in dustier regions of Mars and the thermalinertias of these deposits as determined by TES (298, 278, and 201 Jm�2K�1 s�½ for images I39307007,I36943002, and I41227002, respectively.

4.3. Composition and Origin of Chloride Salt-Bearing Deposits on Mars

The laboratory and modeling work presented here all use halite (NaCl) for the salt fraction in mixtures. Wesuggest that this is the most likely composition for the chloride salt-bearing deposits on Mars. Previous work

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has shown that these deposits are desiccated compared to the surrounding terrain based on the presence ofan inverted 3μm band in CRISM ratio reflectance spectra. These observations do not rule out the presence ofwater in the form of inclusions, which are common in halite. They only constrain the water abundance to beless than that of the surrounding regolith. However, near-infrared spectra of Martian salt-bearing deposits arecharacterized by a featureless red slope with no observable hydration bands [Murchie et al., 2009; Glotch et al.,2010; Jensen and Glotch, 2011; Ruesch et al., 2012].

Felsic or other alkalai-enriched rocks, while present in volumetrically minor abundances on Mars [Christensenet al., 2005; Bandfield et al., 2004b; Bandfield, 2006; Carter and Poulet, 2013; Wray et al., 2013; Sautter et al.,2015; Rogers and Nekvasil, 2015], are not required to provide the necessary Na for chloride salt-bearingregions. Previous studies [Bandfield et al., 2000; Christensen et al., 2000; Rogers and Christensen, 2007] haveall shown that the southern highlands of Mars (where most of the Martian chloride salt-bearing depositsoccur) are composed primarily of basalt with a substantial modeled plagioclase feldspar fraction, rangingfrom ~25 to 65%. None of these studies lists the exact plagioclase composition used, but an intermediatecomposition such as labradorite (Na0.4Ca0.6Al1.6Si2.4O8) is reasonable given the spectral similarities betweenMartian basalts and Deccan Traps flood basalts, which have labradorite as the primary plagioclase phase [e.g.,Bandfield et al., 2000; Christensen et al., 2000; Wright et al., 2011].

Na2O makes up 4.56wt % of labradorite using its empirical formula. Assuming a basalt composed of ~35 vol% labradorite (similar to TES Group 4 of Rogers and Christensen [2007]) and densities of 2.69 and 2.90 g/cm3

for labradorite and basalt, respectively, we can calculate that the basaltic crust of this composition is 1.48weight% Na2O.

Assuming a mixture of 75 vol % basalt and 25 vol % halite, which is consistent with the high end of ourspectral models and a halite density of 2.17 g/cm3, chloride salt-bearing deposits on Mars are ~10.59wt %Na2O. This represents an enrichment of Na2O by only a factor of 7.16. This number, would, of course, go downif the Martian plagioclase composition was more sodic, as has been observed at Gale Crater [Vaniman et al.,2013; Sautter et al., 2015] or the Martian chloride salt deposits had a lower percentage of halite.

Besides the total abundance of Na in the Martian basaltic crust, we must also assess whether Na can be mobi-lized by weathering processes and be available to precipitate as halite or another phase. Hecht et al. [2009]used the Phoenix Wet Chemistry Laboratory to analyze soils at the Phoenix landing site and found that majorcations evolved from the soil were Mg 2+ (3.3mM) and Na+ (1.4mM), with only minor K+ (0.38mM) and Ca2+

(0.58mM). Based on these data, Na+ is one of the most available cations in the Martian soil. This is consistentwith the study of Nesbitt and Wilson [1992], who showed that despite the effects of differing rock composi-tions on leach rates, Na+ is quickly leached and mobilized during weathering of basaltic rocks. Taken insum, these points all suggest that the abundance and availability of Na should not be a limiting factor inthe development of Martian chloride salt deposits.

Sylvite (KCl) is another potential anhydrous chloride phase, but the global mean K abundance in theMartian crust is only 3300 ppm [Taylor et al., 2006] (compared to an abundance of 2.14% in Earth’scontinental crust) [Wedepohl, 1995, and references therein], and substantial concentration of K would needto occur for sylvite to form even as a minor phase in the Martian chloride salt-bearing deposits. While K ismobile and prone to some concentration under relatively neutral pH weathering conditions, we still viewsylvite as an unlikely major phase in Martian chloride salt-bearing deposits given the low initial abundancesin the crust.

Other chloride salts are also unlikely. Molysite (FeCl3) is highly deliquescent, readily transforming to hydromo-lysite (FeCl3 · 6H2O) upon exposure to moisture. Ferrous Fe chloride, rokünite, is also hydrated (FeCl2 · 2H2O)as are the major Mg- and Ca-bearing chlorides bischofite (MgCl2 · 6H2O), sinarjite (CaCl2 · 2H2O), and antarc-ticite (CaCl2 · 6H2O). At the chloride salt content levels required by the THEMIS data, the Martian chloridesalt-bearing deposits would exhibit strong hydration features at VNIR wavelengths. Anhydrous forms of thesesalts are not stable precipitates under the range of temperatures and water activities that were likely presentduring the formation of Martian chloride salt deposits [Davila et al., 2010]. Furthermore, whereas thedeliquescence relative humidity for CaCl2, MgCl2, and FeCl3 ranges from 28 to 45%, NaCl does not start totake up water until a relative humidity of ~75% is reached over a wide range of temperatures [Greenspan,1977; Cohen et al., 1987; Davila et al., 2010].

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Chlorides are not the only chlorine salts that might be considered. Perchlorates have been identified inseveral locations on Mars [Hecht et al., 2009; Kounaves et al., 2010; Cull et al., 2010; Navarro-Gonzalez et al.,2010; Glavin et al., 2013; Ming et al., 2013]. Anhydrous perchlorates are spectrally featureless between 350and 2500 nm, although hydrated perchlorates show strong H2O bands near 1.4 and 1.9μm [Morris et al.,2009; Bishop et al., 2014]. In addition, perchlorates display numerous strong spectral features in the mid-IR[Pejov and Petruševski, 2002; Sutter et al., 2016]. Concentrations of perchlorates at the >5% level would likelyresult in VNIR reflectance and mid-IR emissivity spectra that are inconsistent with the CRISM, TES, and THEMISdata sets. Chlorates, similar to perchlorates, have strong hydration bands at VNIR wavelengths and numerousstrong spectral features at mid-IR wavelengths that would be detectable in THEMIS or TES data at 10–25%abundances [Miller and Wilkins, 1952; Sutter et al., 2016]. Upon consideration of both the geological plausibil-ity and detailed knowledge of the spectral features of the various phases that might contribute to the spectraof the Martian chloride salt-bearing regions, halite is clearly the most likely candidate for the salt fraction ofthese deposits.

4.4. Implications for the Origin of Chloride Salt-Bearing Deposits on Mars

Our work has shown that the spectral features of both the typical coarsely particulate and the newly describedfinely particulate chloride salt-bearing Martian deposits are consistent with chloride salt contents at the10–25wt % level. Here we discuss possible formation mechanisms of these deposits. Halite often forms as avapor phase deposit from volcanic outgassing [e.g., Naughton et al., 1974] and can dominate the surfacemineralogy in the immediate vicinity of volcanic vents, potentially consistent with our observations.However, we view this as an unlikely scenario for the Martian deposits, as to date, no volcanic features havebeen identified in association with them [Osterloo et al., 2010]. Furthermore, deposition by hydrothermal brinesassociated with volcanism, as suggested for Home Plate at Gusev Crater [Schmidt et al., 2008], resulted inchloride enrichment of<2wt %, which is much too small to explain the chloride salt abundances derived fromTHEMIS data.

Freezing can concentrate chlorides in brines. Halite, observed in several SNC meteorites, suggests interac-tions between water and Martian upper crustal rocks and precipitation from cold brines. [Gooding et al.,1991; Bridges and Grady, 1999, 2000]. The abundances of halite in these meteorites, however, are small, withhalite and other evaporite phases filling veins and interstitial pore spaces. This mechanism likely cannotexplain the abundances of halite observed by THEMIS.

Halite, like sulfates discovered at Meridiani Planum and other regions of Mars [Gendrin et al., 2005;McLennanet al., 2005; Murchie et al., 2009], often forms as solute-rich waters or brines evaporate. Halite is expected toprecipitate from SO4-and HCO3- poor brines derived from weathering of basaltic rocks [Tosca and McLennan,2006]. Evaporative environments on Mars are most likely to resemble hydrologically closed continental brinesystems on Earth. These systems display a variety of mineral assemblages, which include chlorides, carbo-nates, and sulfates. The primary factor controlling the composition of the evaporite mineral assemblage ina closed basin is the lithology of the leached rocks contributing to the dilute fluid [Eugster and Hardie,1978]. Eugster and Hardie [1978] defined five fluid types for continental evaporite basins; the evolution ofwhich determines the mineral assemblages within the basin. In each case, carbonates precipitate first, withthe subsequent precipitates determined by the chemistry of the remaining fluid. Tosca and McLennan[2006] argued that the “chemical divides” (turning points in chemical evolution) for precipitating fluids onMars must be different than those that occur on Earth. This is because Martian fluids primarily leached maficand ultramafic crust—a condition that is relatively rare for terrestrial evaporite systems. Therefore, Tosca andMcLennan [2006] reevaluated chemical divides and brine evolution for Martian systems and defined fivebrine evolution pathways. Of these, three result in late-stage brines favoring chloride salt precipitation. Ineach case, the pathway to the formation of these late-stage brines and precipitates includes the precipitationof carbonates (calcite, siderite, and/or magnesite) and sulfates (gypsum, melanterite, and/or epsomite). Noneof these phases has been found in association with Martian chloride salt-bearing deposits, leading Rueschet al. [2012] to suggest SO3-Cl brine fractionation [e.g., Moore and Bullock, 1999] as a possible mechanismfor forming the Cl-rich brines required for the Martian deposits. This mechanism has been observed inAntarctic permafrosts, resulting in the spatial separation of halide and sulfate deposits [Dickinson andRosen, 2003]. Another possibility is that additional evaporite phases do occur in these regions but are (1) pre-sent at low enough abundances (likely less than a few percent) to remain undetected in both VNIR and MIR

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data or (2) buried by the chloride salt deposits and undetectable. Recent laboratory work has shown thatthick coatings of halite can obscure the spectral signatures of materials underneath at MIR wavelengths[Berger et al., 2015], although this has yet to be verified at VNIR wavelengths.

Although halite is often found in association with other evaporite phases in lacustrine or playa depositionalsettings [e.g., Lynch et al., 2015, and references therein], its presence as a major phase in these settings isconsistent with the inferred abundances for the Martian deposits. Still, the apparent lack of additional evapor-ite phases, either due to geochemical fractionation or optical masking effects, must be addressed whencomparing the Martian deposits to terrestrial lacustrine evaporative environments. This work also shows that~75–90% of the chloride salt deposits are composed of a silicate component that is spectrally consistent withthe local basaltic regolith or, in three cases, the globally homogenous dust. Aeolian input of the silicate mate-rial, either through sand saltation or dust suspension and deposition, into a lacustrine/playa environmentcould explain the substantial silicate fraction. Alternatively, pervasive groundwater upwelling into a porousbasaltic regolith could explain the observed spectral and thermophysical features.

5. Conclusions

We have shown that THEMIS observations, supported by TES, CRISM, and OMEGA, of chloride salt-bearingdeposits on Mars are consistent with ~10–25% halite mixed with the regional Martian regolith. The spectralcharacter of the chloride-bearing materials displays significant differences that are dependent on the particlesize and abundance of the intermixed materials. Where regional coarse particulates are present, THEMIS datashow a concave up spectral shape superimposed on a blue spectral slope. Where fine-particulate dust ispresent, THEMIS spectra of these deposits are characterized by a concave down spectral shape, similar tosilicate transmission spectra, superimposed on a blue slope.

We have used spectra of physical mixtures of halite and flood basalt and a hybrid T-matrix/Hapke scatteringmodel to constrain the particle size and abundance of halite in the Martian chloride salt-bearing deposits.When convolved to THEMIS spectral sampling, laboratory and modeled spectra of the coarsest particulatesand halite contents <10% or> 25% fail to match typical THEMIS spectra of these units. Rather, particle sizesbetween 63 and 180μm and halite contents between 10 and 25% are consistent with the THEMIS data inregions with coarse particulate surfaces. In the cases with regional fine-particulate materials, particle sizes<10μm and abundances of 10 to 25% are required to match the THEMIS data. These sites all occur inmoderately dusty regions on Mars. This work has also shown that the position of the emissivity maximumin THEMIS spectra of chloride salt-bearing deposits is controlled by the salt abundance, with an emissivitymaximum in band 3 (centered near 1270 cm�1; 7.89μm) indicating a relatively low abundance<~10%and an emissivity maximum in band 4 (centered near 1170 cm�1; 8.51μm) indicating higher abundancesbetween 10 and 25%.

The abundances and composition of chloride salt derived in this work help to constrain the formationmechanism(s) of the deposits. Hydrothermal brine deposition or cold brine precipitation is unlikely, asobserved chloride salt contents attributed to both of these processes are much too low to explain theobserved THEMIS data. A major outstanding question is the apparent lack of other major evaporite phases,including sulfates, phyllosilicates, and carbonates, associated with most of the deposits. The data presentedhere are most consistent with the presence of halite as a major phase deposited in either a lacustrine/playasetting or in association with late-stage groundwater upwelling.

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